A hydraulic excavator as shown in FIGS. 13A and 13B is known as a construction machine for excavating and loading earth and sand, which includes: a traveling hydraulic motor 1 for traveling a lower traveling body a; a swing hydraulic motor 2 for swinging an upper swing body b; work equipments c (a boom 3, an arm 4 and a bucket 5) mounted on the front side of the upper swing body b; and a boom cylinder 6, arm cylinder 7 and bucket cylinder 8 for driving the work equipments c.
The hydraulic excavator performs a sequence of operations during an excavating process such as excavation, lift-up swing, earth removal and lift-down swing. Especially during the lift-up swing, while the boom is lifted up and the arm is dumped (as shown in FIG. 13A), swing operation is conducted toward a loading platform of a dumper truck d that is stopped around forty-five, ninety or one-hundred-eighty degrees from the excavated point (as shown in FIG. 13B).
When the swing and boom movements or the swing and arm movements are concurrently conducted, pressure in swing circuit is influenced by boom circuit or arm circuit, where, if the boom circuit or the arm circuit is in low pressure, the swing circuit is also in low pressure, so that smooth swing movement may not be conducted.
Further, since the swing angle differs according to the stop position of the dumper truck d, an operator controls spool-opening degree of flow control valve of hydraulic actuators for each operation to control supply flow rate toward respective circuits to match both of the movements. For instance, the supply flow rate to the respective circuits is controlled so that: when the dumper truck d is stopped at forty-five degrees position from the excavated point, the boom-lift movement is accelerated and swing speed is decelerated; and, when the dumper truck d is stopped at one-hundred eighty degrees position from the excavated point, the swing speed is accelerated and boom-lift speed is decelerated, thereby matching both of the movements. However, such operation is very difficult and exhausting.
Accordingly, the Applicant of the present application has proposed a hydraulic control circuit of a hydraulic excavator for resolving the above problems (Patent Document 1).
Specifically, the hydraulic control circuit of a hydraulic excavator includes: hydraulic actuators respectively for boom-lifting, arm-lifting and swinging movement; boom control lever; arm control lever; swing control lever; hydraulic circuit for driving the hydraulic actuators based on the operation on the control levers; and an operation-mode selecting switch. When one of boom-priority mode, swing-priority mode and standard mode is selected by the operation mode selecting switch, pressure oil is preferentially flowed to the hydraulic circuit corresponding to the selected mode.
[Patent Document] Japanese Patent Publication No. 2583148